Devices for electroimpedance tomography (EIT) are known from the state of the art. These devices are designed and provided for generating an image, a plurality of images or a continuous sequence of images from signals obtained by means of electroimpedance measurements and by means of data and data streams obtained therefrom. These images or sequences of images show differences in the conductivity of various body tissues, bones, skin and bodily fluids (blood, lymphatic fluid, cerebrospinal fluid) and organs (lung, heart), which are useful for a diagnosis of diseases, clinical pictures.
U.S. Pat. No. 6,236,886 describes an electroimpedance tomograph containing an array of a plurality of electrodes, feeding of current to at least two electrodes and a method with an algorithm for image reconstruction for determining the distribution of conductivities of a body, such as bones, skin and blood vessels in a basic design with components for detection of signals (electrodes), processing of signals (amplifier, A/D converter), feeding of current (generator, voltage-current converter, current limitation), components for control (mC). The electroimpedance tomograph makes possible a visualization of changes in conductivity within a heart beat and the monitoring of the blood streams in the heart and in the vessels, as well as time dependencies in the perfusion of heart regions in the form of an impedance cardiogram with additional information on the heart function. By means of this visualization, further application possibilities are possible for recognizing internal hemorrhages, external inflammations, examinations on digestive organs, tumor monitoring, inflammations of the breast and various types of diseases of the lung. Moreover, a monitoring of changes in temperature of internal organs is possible.
In U.S. Pat. No. 5,807,251, it is explained that it is known in the clinical application of EIT to provide a set of electrodes, which are arranged at a defined distance from one another, for example, about the thorax of a patient in electrical contact with the skin, and to apply an electric current or voltage input signal each alternatingly between various or all of the possible pairs of electrodes arranged adjacent to one another. While the input signal is applied to one of the pair of electrodes arranged adjacent to one another, the currents or voltages between each pair of the remaining electrodes arranged adjacent to one another are measured and the measurement data obtained are processed in the known manner to obtain a visualization of the distribution of the specific electric resistance over a cross section of the patient, about which the ring of electrodes is arranged, and to display same on a display screen.
It is known from U.S. Pat. No. 5,272,624 to use a medical electroimpedance imaging method using set current patterns, which are fed into the feeding electrodes.
U.S. Pat. No. 5,184,624 shows an array of a plurality of electrodes for an electroimpedance measurement on a body, with feeding of electric current via a pair of electrodes into the body and detection of voltage potentials at the body via the remaining pairs of electrodes. Two electrodes each from the plurality of electrodes, which are arranged in the manner of a circulation about the body one after the other, are thereby selected for the feeding pair of electrodes, and a plurality of the remaining electrodes are used as pairs of electrodes for detecting the voltage potentials.
In U.S. Pat. No. 4,486,835 is described a device for electroimpedance tomography, with which the feeding of electric signals to selected first pairs of electrodes and the detection of electric signals at selected second pairs of electrodes with a preset sequence by means of a multiplexing device is carried out on a body, and is then forwarded to a calculation device coupled to the multiplexing device for the determination of electric properties of a plurality of local regions of the body. These local regions are thereby classified into a three-dimensional imaging grid structure in the body and in an iterative process, the electric conductivity at the plurality of local regions is continuously updated.
U.S. Pat. No. 5,311,878 describes an electroimpedance tomography method and device for real-time imaging. Electroimpedance measured signals from two selected adjacent electrodes are fed to a digital signal processing by electrodes arranged around a thorax with simultaneous feeding of a feeding current in selected electrodes and real-time images are generated by means of computer-based reconstruction.
Besides devices for electroimpedance tomography (EIT), other medical devices that are suitable for imaging, for example, the widest variety of radiological devices, such as X-ray devices, computer tomographs (CT), nuclear magnetic resonance (NMR) devices, nuclear spin or magnetic resonance tomographs (MRT, MRI), as well as also sonographic devices for cardiological, angiological, as well as prenatal or neonatal imaging, which make possible a real-time imaging and a providing of signals or image data, in the area of health care, are being used.
U.S. Pat. No. 3,310,049 A describes a method for determining the heart volume with ultrasound.
An ultrasound-pulse Doppler device for the sonographic determination of the functional capacity of the heart (cardiac output) is known from U.S. Pat. No. 5,052,395 A.
Thus, an X-ray tomography system based on gamma radiation is known from U.S. Pat. No. 4,075,482 A.
U.S. Pat. No. 4,806,867 A shows a magnetic resonance imaging system.
A device for improved image reconstruction of computer tomograms is described in U.S. Pat. No. 4,149,081 A.
A contrast medium, which is introduced into the patient invasively via an access, is usually used in a radiological-cardiological examination by means of such computer tomographs (CT) according to the state of the art. Such a contrast medium, especially when a radioactive isotope is used thereby, represents an extraordinary physical burden for the body and thus makes a continuous imaging monitoring of vital functions of the heart and lung impossible. For generating analyzable images on the condition of the coronary vessels of the heart, it is, moreover, necessary to take the tomographic pictures in the rest phases of the heart or to determine the corresponding useful data by means of reprocessing the tomographic pictures. The tomographic pictures are combined in this case in connection with an electrocardiogram (ECG) by means of two proven methods, the so-called “prospective ECG triggering” or the so-called “ECG gating.” In “prospective ECG triggering,” the rest phases of the heart are determined by means of ECG and synchronized with the tomographic pictures, in which the patient is then transilluminated layer by layer by table feed. Thus, no real-time total picture of the heart takes place at the same time, but rather the individual pieces of information obtained layer by layer are put together later. In “ECG gating,” a tomographic real-time picture of the entire heart and at the same time a continuous detection of ECG signals are taken in a so-called “spiral CT,” in which the rotation of the tubes and the table feed take place at the same time. The tomographic pictures are processed by means of the “ECG gating” with inclusion of the ECG signals after the examination and analyzable images are generated therewith. A drawback in both methods is that the computer tomograph (CT) additionally needs an ECG signal as an external signal for synchronization; moreover, it is especially disadvantageous in “ECG gating” that the patient may not breathe during the tomographic examination by means of spiral CT and that the reprocessing of the acquired data is very time-consuming, and thus the result is not available during the examination itself, but rather only delayed in time. Also in the use of magnetic resonance tomographs (MRT), both a synchronization with an electrocardiogram (ECG) for combining data from a plurality of cardiac cycles into complete images and a use of contrast media are usual in order to improve the resolution and the contrast of the tomographic pictures. These exemplary embodiments for imaging by means of MRT or CT in the cardiological environment are applicable to the imaging (angiology/angiography) of the lung by means of MRT, CT in the basic sense.
Moreover, these exemplary embodiments clearly show that devices for electroimpedance tomography (EIT) in comparison to magnetic resonance tomographs (MRT) and also computer tomographs (CT) have marked advantages and essentially regarding the following aspects:
EIT has a real-time functionality
EIT does not require coupling to ECG
EIT does not require contrast medium.
As for sonographic devices for cardiological, angiological, prenatal or neonatal imaging, it should be noted that these devices are limited in use to temporary examinations, since the transducer must be guided in connection with the contact gel by the user and the alignment of the transducer, as well as the function of the contact gel must be continually visually observed by the user during the examination. The following advantages arise from this:
EIT does not require contact gel
EIT does not require continuous monitoring of function by the user.
Thus, electroimpedance tomography (EIT) is, unlike the other medical devices suitable for imaging (X-ray devices, computer tomographs, magnetic resonance tomographs, sonographic devices) cited, suitable for a continuous and long-lasting imaging, especially of the lung as well as the lung and heart, without causing a considerable physical burden or discomfort for the patient.
EP 1 292 224 B2 describes a method and a device for the visualization of data that were obtained by means of electroimpedance tomography. Various special modes for analysis are described, on the basis of which an analysis of the condition of a lung of a patient is provided. Thus, a relative mode is provided, which processes regional changes of a two-dimensional distribution of the ventilation for a past period. Furthermore, a perfusion mode is provided, which processes a two-dimensional distribution of the lung perfusion in a past period of a cardiac cycle. Moreover, a phase shift mode is provided, which processes a dynamics of the ventilation. Additional modes described in this EP 1 292 224 B2 are absolute mode, time constant mode and a regional spirometry mode. The various modes are used for distinguishing various lung conditions. One of the modes or a plurality of these modes is/are selected one after the other for operation for distinguishing various lung conditions. It is common to all modes described in this EP 1 292 224 B2 that no modes and no combination of modes are provided, which makes possible or make possible a shared visualization or processing of perfusion and ventilation.
In the respiration of a patient, especially in intensive care, it is of central importance that the lung of the patient be both ventilated and perfused as best as possible. For, only if an as good as possible perfusion and an as good as possible ventilation over as variable as possible regions of the entire available lung volume is given can the gas exchange, i.e., the introduction of oxygen from the lung into the blood circulation and the transfer of carbon dioxide from the blood circulation via the lung out of the patient take place effectively.
Depending on the constitution, medication and clinical picture of the patient and depending on the settings of the ventilation on a ventilator (also known as a respirator), by means of which the patient is ventilated, the following different physical basic constellations A-D in various local lung regions arise without direct relation to special disease conditions (e.g., atelectasis, emphysema, pneumonia, embolism) in the lung of the patient:
Constellation A: Lung regions with sufficient ventilation and with sufficient perfusion,
Constellation B: Lung regions with sufficient ventilation and with insufficient perfusion,
Constellation C: Lung regions with insufficient ventilation and with sufficient perfusion,
Constellation D: Lung regions with insufficient ventilation and with insufficient perfusion.
This distinction into four constellations in the lung is a common simplified classification in the state of the art, as it is also carried out in EP 1 292 224 B2.
EIT is able, in a spatially resolved manner, to differentiate between perfusion and ventilation from the impedance differences between air/gas and blood. A plurality of heart beat cycles are present in a breath of a patient at the same time. Blood flows into the lung and also out again with each heart beat.
The heart beat cycles have a certain variability in the heart rate and are asynchronous to breathing and different from the respiration rate.
If an EIT sequence of images of local impedance changes or impedances of various regions of the lung for one or more breaths is now observed as a type of overlay consisting of local cardiac- and perfusion-related impedance changes (signals) CPRS (Cardiac and Perfusion Related Signal) and local ventilation-related impedance changes (signals) VRS (Ventilation Related Signal), then the blood flow (perfusion-related impedance changes) from the lung to the heart and from the heart to the lung overlays the breathing (ventilation-related impedance changes).
The cardiac- and perfusion-related impedance changes (CPRS) are thereby divided into cardiac-related impedance changes (signals) CRS (Cardiac Related Signal), which are essentially based on the respective degree of filling of the heart and perfusion-related impedance changes (signals) PRS (Perfusion Related Signal), which are based on the blood distribution (pulmonary perfusion) in the lung tissue.
The result of the overlay between VRS and CPRS, or PRS is that in the sequence of images it is not possible to visually recognize how the constellation regarding perfusion (PRS) and ventilation (VRS) in a local lung region is in each case. In addition, the visual recognizability is made more difficult, on the one hand, by impedance differences of lung regions filled with air compared to the surrounding tissue (muscles, skin, bones) being markedly more variable than the impedance differences of lung regions filled with blood compared to the surrounding tissue (muscles, skin, bones) and thus the CPRS compared to the VRS has a markedly lower signal. In addition, the visual recognizability and quantitative analysis of the perfusion is made more difficult by the phase shifting of the blood flow due to the pulse transit time between heart and lung tissue. The human eye and in connection therewith the cognitive attention naturally follow a movement in the image, in this case the progression of the blood flow in the sequence of images and are thereby at the same time hardly able to determine the strength and its changes of a signal at a defined location over a plurality of images of the sequence of images.
A location-related observation of various lung regions at an observation point in time over time in relation to the beginning of the pulse wave at the heart reveals in a comparison of the perfusion signals of two different regions (region A, region B) of the lung that in the one region (region A) the perfusion is given with a certain quantity, while at the same observation point in time in a different region (region B) the perfusion thereof is quantitatively markedly different. This is explained in that the pulse wave originating from the heart does not reach the various regions of the lung at the same time, but rather takes place delayed differently over time with propagation of the pulse wave into the blood vessels of the lung depending on the vessel length and on the vessel properties (flow resistance, elasticity). This delay becomes apparent as a phase shift between the perfusion signals.
This phase shift is perceived in the EIT sequence of images in a visual analysis, so to speak, as a “progression of the blood flow,” which in reality takes place in the lung tissue in this form, but neither in real time nor quantitatively over time in this form. Therefore, adding up the quantities of a plurality of signals without including the phase information at a point in time over a plurality of regions does not produce a quantitative overall image of the perfusion over a plurality of regions of the lung. Partly due to these phase differences in the blood flow between various regions of the lung, an adding up of perfusion signals of a plurality of lung regions thus produces a partial extinction of the perfusion signals with one another, such that no utilizable quantifiable statement on the perfusion of the entire lung or regions of the lung is possible in this way. Also, an adding up over time of a plurality of perfusion signals at a single location or a plurality of defined locations, for example, over a breath or a plurality of breaths, parts of a breath, such as inspiration and expiration or a plurality of breaths is not suitable to generate local images, from which differences in the perfusion between various regions of the lung are clearly and unambiguously obvious. In an adding up over one breath, for example, an adding up over a plurality of heart beat cycles arises, on the one hand, such that for a defined location, the perfusion in one breath fluctuates repeatedly and thereby blood flows in and then flows out again at this defined location, such that the perfusion also cannot, on average, be quantitatively determined for this single location.